This invention is directed generally to flow control valves and more specifically to cage-guided control valves.
U.S. Pat. No. 4,256,284 describes a high energy loss fluid flow control device that uses a stepped-spiraled-tapered bore and control element to minimize clearance flow. The ""284 patent, however, presents a clearance flow minimizing design that is physically and operationally different from the invention. The ""284 patent design does not address specific geometry for controlling the pressure recovery of the controlled fluid and it does not allow for high rangeability.
A typical globe valve is a valve with a linear motion closure member, one or more ports and a body typically distinguished by a globular shaped cavity around the port region. The body is the part of the valve which is the main pressure boundary. The body typically provides the pipe connecting ends and the fluid flow passageway. In a globe valve, the closure member is a movable part of the valve that is positioned in the flow path to modify the rate of flow through the valve.
A plug closure member is a part, often cylindrical in nature, which moves in the flow stream with linear motion to modify the flow rate. It may or may not have a contoured portion to provide flow characterization. It may also be a cylindrical or conically tapered part, which may have an internal flow path, that modifies the flow rate with rotary motion. Other closure members include ball, disk and gate, for example.
A flow orifice in the flow passageway (path) interacts with the closure member to close the valve. The orifice may be provided with a seating surface, to be contacted by or closely fitted to the closure member, to provide tight shut-off or limited leakage, i.e., to close the valve.
A cage is typically a part in a globe valve that generally surrounds the closure member to provide alignment and facilitate assembly of other parts of the valve trim. The cage may also provide flow characterization and/or a seating surface. Globe valve trim typically includes the internal parts of a valve which are in flowing contact with the controlled fluid. Examples of valve trim are the plug, seat ring and cage. The body is not considered part of the trim.
FIG. 1 depicts a standard cage-guided globe control valve 10, comprising a body 12, that controls flow 14 by modulating the valve plug 16 to expose holes 18 (ports or flow paths) of a port region 19 in the valve cage 20. Controlling surfaces are surfaces that define an area that throttles the process fluid, i.e., the surface is subject to the application of pressure differential. Other surfaces may be in contact with the fluid, but are not actively involved in the throttling process. The controlling surface is either between the cage port 18 and plug 16 (FIG. 2) or the seat ring 22 and plug 16 depending upon plug 16 position within the cage 20.
To obtain high flow capacities through such a valve, large size seat rings, cages, and plugs are utilized. To allow free movement of the plug 16 within the cage 20, a radial clearance 24 must be provided between the plug outside diameter 26 and the cage bore 28 (FIG. 3). This radial clearance 24 typically increases with increasing valve size. As the plug 16 is lifted off of the valve seating surface 30, and before the initial cage port area 32 is exposed, the radial clearance area 24 is allowed to pass fluid flow 34. The flow 34 transmitted through this radial clearance area 24 is the minimum flow in the valve 10. The clearance flow area 24 is important in this type of valve design because this area 24 will limit the minimum controllable flow as defined by the valve characteristic.
The flow characteristic of a cage throttling control valve 10 is defined by the cage port throttling area exposed (see, for example, FIGS. 2 and 3, items 38 and 18, respectively). Throttling below the cage port area causes the entire throttling pressure drop to occur in the clearance flow area 24 (i.e., flow through the guide clearance). Under high-pressure drop conditions, particularly high-pressure liquid letdown (as opposed to gas), clearance flow throttling can be very damaging to metal surfaces, thereby causing erosion 36. Some cage throttling valves 10 will utilize multiple pressure drop staging 38 throughout the cage flow area, however, the staged flow ports (40 in FIG. 2) located above the throttling area 42 have direct communication to the valve outlet (orifice) 44 through the guide clearance area 24. This will lead to full pressure drop between the non-exposed staged flow paths and the exposed flow paths (see FIG. 2). This can ultimately lead to erosion 36 in the guide clearance 24 due to cavitation and high-velocity wear, resulting in eventual valve-clearance vibration problems. Continued erosion will increase the guide clearance and further exacerbate vibration-related problems.
Valve manufacturers define the ratio of maximum controllable flow to minimum controllable flow as the valve rangeability. Most cage-guided globe control valves have a rangeability, limited by the clearance flow, to approximately 100:1.
Accordingly, it is an object of the invention to provide a valve with rangeability greater than approximately 100:1. A further object is to provide a valve with rangeability greater than approximately 500:1.
Another object is to provide a valve that reduces or eliminates cavitation erosion damage to seating areas.
Another object is to provide a valve with a pressure recovery area, away from the seating area.
Yet another object is to provide a valve with increased rangeability that reduces or eliminates cavitation damage on seating surfaces.
Another object is to provide a valve with a clearance seal.
Yet another object is to provide a valve with a clearance seal and a pressure recovery area.
Other objects and advantages will be apparent from the teachings herein.
Briefly, in accordance with the foregoing, a control valve comprising a vapor recovery area addresses problems associated with the prior art. A seal is preferably positioned above the vapor recovery area to reduce flow between a closure member and alignment means for the closure member.
In an embodiment of a globe control valve, a plug is aligned in a cage to contact a seat. The vapor recovery area is positioned between the cage and the plug. Preferably, at least when the plug is opened, the vapor recovery area is above the seat. The seal reduces flow through a clearance between the cage and the plug. For some applications, the seal comprises an outer seal contacting the cage and an inner seal between the outer seal and the plug.
More generally, a control valve is provided with means for encouraging collapse of vapor bubbles. In a particular embodiment, a closure member interacts with an orifice to close the valve. A clearance is defined between the closure member and a means for aligning the closure member. Means for reducing flow through the clearance comprises, for example, a seal coupled to move with the closure member.
More specifically, a control valve may be provided with a linearly movable plug positioned in a fluid flow path. The plug moves in a cage comprising a plurality of ports located axially along the cage. Thus, moving the plug will modify the rate of fluid flow. A radial clearance is defined between the cage and the plug. And a seal, comprising an elastomeric inner seal and metallic outer seal, reduces flow through the clearance. The plug defines, at least in part, a vapor recovery gallery between the plug and the cage, wherein the gallery is below the lowest port when the plug is seated. When the plug is lifted to expose, at least in part, the lowest port, the gallery is above a seat that interacts with the plug to close the valve.
It will be understood that use of directional terms such as above and below are for convenience only and not intended to limit the scope of the teachings or invention claimed herein. Generally an above position is upstream, opposite the direction of fluid flow, of a down position.